1. Field of the Invention
The present invention relates generally to an optical repeater applicable to a system requiring high reliability, such as an optical submarine repeating transmission system, and more particularly to an optical repeater having redundancy. The wording of "having redundancy" used herein means having a capability of performing the function instead in the event that a device or a part of a system requiring high reliability has failed.
2. Description of the Related Art
An optical submarine repeating transmission system is a system for transmitting a light signal by laying a cable including an optical fiber transmission line in a submarine section between the continents or the like, wherein the light signal attenuated is repeated by optical repeaters placed on a sea bed to thereby compensate for attenuation or the like. In recent years, an optical repeater having an optical amplifier has been put to practical use, and it has been desired to improve the reliability of the optical repeater and reduce the cost thereof.
Known as the optical amplifier used in the optical repeater is an optical amplifier including a doped fiber doped with a rare earth element such as Er (erbium), a pumping light source for outputting pumping light, and an optical coupler operatively connected to either a signal light input end or a signal light output end of the doped fiber, for supplying the pumping light to the doped fiber. When the optical coupler is operatively connected to the signal light input end of the doped fiber, the signal light and the pumping light propagate through the doped fiber in the same direction, whereas when the optical coupler is operatively connected to the signal light output end of the doped fiber, the signal light and the pumping light propagate through the doped fiber in opposite directions. In both cases, the doped fiber is pumped by the pumping light. When the signal light is supplied to the doped fiber in its pumped condition, the signal light is amplified. In the case where the optical repeater is applied to an optical cable including two, up and down optical fiber transmission lines, two optical amplifiers, one for the up optical fiber transmission line and the other for the down optical fiber transmission line, are incorporated in the optical repeater.
In general, a laser diode usually employed as the pumping light source is less reliable than a passive optical component such as an optical coupler. Accordingly, the pumping light source is conventionally duplexed, so as to provide an optical amplifier having high reliability. In starting the operation of the optical amplifier, pumping light from a first laser diode is supplied to the doped fiber. When the first laser diode has failed during the operation, a second laser diode is selected in place of the first laser diode, and output light from the second laser diode is supplied as pumping light to the doped fiber. Since both the first and second laser diodes must be connected to the doped fiber, a polarization beam splitter is used to couple the beams of linearly polarized light output from the first and second laser diodes to one optical path. Of the components constituting an optical circuit of the optical amplifier, the laser diode is highly expensive, and the polarization beam splitter is also expensive next to the laser diode. Accordingly, the duplexing of the pumping light source substantially doubles the cost of the optical amplifier.